Abstract

Chronic IL-6 signalling contributes to the pathophysiology of many diseases including prostate cancer. Relevant to prostate cancer is the ability of the pro-inflammatory cytokine IL-6 to activate the oncogenic signalling protein STAT3, thus inhibition of STAT3 activation is a popular avenue of research to augment prostate cancer therapies. In this study, the endogenous anti-inflammatory molecule cAMP was investigated as a mechanism by which to inhibit IL-6-induced STAT3 activation in the DU145, LNCaP and
PZ-HPV-7 prostate epithelial cells.
Elevation of cAMP attenuated IL-6-mediated activation of STAT3 which was mimicked via selective activation of the exchange protein activated by cAMP. Inhibition of protein
kinase A (PKA) alone also attenuated IL-6-induced STAT3 activation, suggesting a role for PKA activity in sustained IL-6 signalling in these cells. In DU145 and PZ-HPV-7 cells,
the inhibitory effect of cAMP elevation was correlated with an increase in protein levels of suppressor of cytokine signalling 3. However, this was not the case in LNCaP cells in which cAMP elevation was instead associated with morphological changes consistent with neuroendocrine-like differentiation associated with terminal disease.
PKA activation was required for cAMP-mediated changes in LNCaP cell morphology and could be recapitulated by reagents which inhibited RhoA/ROCK signalling, suggesting that cAMP elevation is able to inhibit RhoA activation via a PKA-dependent pathway. Additionally, cAMP elevation activated ERK1/2 and selective blockade of ERK signalling
attenuated the effects of cAMP elevation on cell morphology. Selective activation of ERK1/2 did not induce the early changes in cell morphology associated with increased intracellular cAMP concentrations, suggesting that another, related pathway was responsible for this phenomenon. Genetic or pharmacological inhibition of the
MEK5/ERK5 signalling pathway significantly attenuated the rapid cAMP-mediated changes in LNCaP cell morphology, suggesting this pathway may be a possible target by which to inhibit the onset of neuroendocrine differentiation.
To summarise, this study demonstrates that whilst the ability of intracellular cAMP elevation to inhibit STAT3 activation is common to the prostate epithelial cell lines used, the downstream effects of cAMP elevation can vary dramatically. Thus, whilst modulation of cAMP signalling may represent a suitable therapeutic strategy when considering some aspects of prostate cancer, the impact on other signalling events must be considered.